all things considered, the premise of STO kinda lends itself to allowing feds to have cloak. if memory serves me right, the federation is assisting the RSE to rebuild (which is why we are able to enter romulan space) while the klingons are making a push for Romulan territory. while (some) romulans do harbor a resentment towards the federation for the inability to help, much of the fights with the romulans have been involved in uncovering very cloak and dagger type anti federation movements. given the federations public stance of assisting the romulans, it wouldnt be a far stretch in the story for romulans to say something to the equivalent of "because you are assisting us in rebuilding our shattered empire and helping fight off the klingons, we can bend the rules on the treaty of algeron to allow you cloaking during these difficult times."